Up to now, ethanol has been prepared by chemical synthesis via ethylene from fossil resources such as coal and petroleum, or a fermentation of sugars from biomass resource such as plant by a microorganism such as yeast or bacteria. Among these, a process for producing ethanol by fermentation using a renewable biomass resource in light of energy resource or environmental problems has been noticed.
A process for producing ethanol by a conventional large-scale industrial fermentation is a process by fermenting starch or sugars from various biomass resources, namely, a technology based on a brewage for potable ethanol. However, the use of a yeast as a fermentation microorganism results in slow rate of ethanol production and also difficulties such as complicated fermentation control because of the necessity of aeration, even though the fermentation is performed under anaerobic condition.
It is well known to use a strain belonging to Zymomonas as a fermentation microorganism (Japanese Patent Publication No. H7-59187).
It is recognized that a fermentation process by using Zymomonas leads to increased rate of ethanol production compare to that by using a yeast. Improvement of ethanol production efficiency by biotechnological modification of various microorganism species has been proposed (Japanese Patent Publication Nos. H5-502366, H6-504436, and H6-505875).
In the above technologies, 2 genes from Zymomonas mobilis, one encoding for PDC activity (which catalyses the conversion of pyruvic acid in the glycolytic pathway to acetoaldehyde) and another ADH activity (which catalyses the conversion of acetoaldehyde to ethanol) were inserted into host (enteric bacteria such as Escherichia coli or Erwinia chrysanthe) under a regulatory sequence allowing for their expression. Consequently, ethanol fermentation with high productivity was achieved by using such transformants.
According to the abovementioned technologies, improvement in ethanol productivity owes to the high growth and high cell density of the transformants in the fermentor. Such fermentation involving cell growth presents many inefficiencies including low conversion efficiency of sugar material to ethanol because the sugar material is used as an energy supply source for growth of said microorganism, low ethanol production rate during the period until the microorganism reaches stationary growth phase and complications in fermentor control due to the change of density of the microorganism accompanying the cell growth thereof.
Another technological problem is separation of toxic substances when Escherichia coli used as a host cell. Escherichia coli is succeptible to bacteriolysis leading to possible contamination of fermentation products with toxic intracellular protein.
From this point of view, improvement of host microorganism to be transformed is desirable.
Problem to be Solved by the Invention
The present invention provides a new process for producing ethanol by using biomass resources as raw material for sugars, wherein better excellent technology for efficiently producing ethanol at a high productivity has been accomplished and wherein various problems pointed out in the foregoing prior art have been solved.
Means to Solve the Problem
The present inventors have made extended studies to solve the problems mentioned above, and have found that efficient production of ethanol at a high productivity can be achieved by using a coryneform bacterium which has been transformed with a gene expressing PDC activity, and if desired, a gene expressing ADH activity under a regulatory sequence allowing for expression, and then, performing fermentation under ethanol production conditions wherein the transformed coryneform bacterium does not substantially proliferate, whereby the present invention has been accomplished.
One of the characteristics of the present invention is that a host bacterium to be transformed is a coryneform bacterium and that ethanol is prepared under conditions wherein the transformed coryneform bacterium does not substantially proliferate.